Vane type positive displacement pumps and motors use mechanical power to compress a fluid when operating as a pump and compressed fluid as a power source when operating as a motor. To avoid complication of description, these devices will be described herein as pumps, it being understood that the reverse operation as a motor is equally possible.
Typically these pumps comprise a housing containing a liner with a bore and a pair of end bearings which support a rotor shaft with its axis parallel to but offset from the axis of the liner. Vanes or blades slide radially in and out in slots through the shaft to define pockets which expand and contract with each shaft revolution.
Axial positioning and sealing of the ends of the vanes or blades of prior art pumps have been accomplished in different ways, each with accompanying advantages and disadvantages. One technique permits maximum volumetric efficiency but limits design and construction flexibility, whereas another technique has reduced volumetric efficiency but enhanced design flexibility. In this context, volumetric efficiency is increased when the rotor shaft is mounted as nearly tangent to the liner bore as practically possible. Also, mechanical integrity is enhanced when multiple pump units are mounted on a single rotor shaft.
The first of these prior art techniques involves the provision of stepped journals on the rotor shaft at opposite ends of the liner. This permits the end bearings that are mounted on the shaft journals to overlap radially the shaft outer diameter surrounded by the liner (hereinafter sometimes referred to as the "rotor barrel"), thus serving to position the vanes or blades axially even when the innermost retracted position of the blade tips substantially corresponds to the outer diameter of the rotor barrel. However, this design prohibits more than one pump unit per shaft because the end bearings for the pump unit must be assembled from opposite ends of the shaft. Accordingly, to provide a multiple pump unit utilizing this design requires interconnecting the shafts of a plurality of individual pump units using various types of drive couplings, thereby compromising the mechanical integrity of the pump design and increasing the overall cost.
Another prior art technique for axial positioning of vanes or blades in this type of pump utilizes a shaft of uniform diameter which permits multiple pump units to be mounted on a single shaft. However, this is accomplished at the expense of volumetric efficiency, in that in order to position the vanes or blades axially when the blade tips are at their innermost retracted positions, the rotor barrel is mounted short of tangent to the liner bore. Thus, the blade tips never retract completely, leaving as a minimum a small amount of the end faces of each blade projecting radially outwardly beyond the outer diameter of the rotor barrel which bear against a respective end bearing to position the blades axially. Because the rotor barrel is not tangent to the liner bore, the blades never retract completely into the barrel, whereby volumetric efficiency is reduced.
Still another prior art technique which permits multiple pump units to be mounted on a single shaft is disclosed in U.S. Pat. No. 4,619,594. In this patent, which is assigned to the same assignee as the present application and is incorporated herein by reference, the axial positioning of the blades (vanes) is accomplished by providing each blade with a radially extending tab at one axial end thereof. The tabs fit in an annular groove formed either by a counterbore in each pump liner or by a separate wafer at one end of each pump liner having an internal diameter that is greater than the liner internal diameter. While the volumetric efficiency of this pump design is greater than the prior art technique of mounting the rotor barrel short of tangent to the liner bore, it still lacks the volumetric efficiency of the single pump unit per shaft design previously described because of the small volume of fluid, termed the carryover volume, in the pockets in the counterbore between the tabs which in effect is never expelled from the pump.